First by using domesticated animals and then by using machines (e.g., planes, trains, and automobiles), humans have sought ways to reduce the weight they must personally carry. Even so, humans seem destined to continue to transport considerable weights of food, clothing, medicines, books and tools wherever they go. On a smooth, even surface, such as an airport terminal, people can use wheels to move their loads from one place to another. On uneven terrain or in the wilderness, however, small wheels are ineffective. Hence, the ubiquitous backpack remains the preferred solution to move loads around with us.
Backpacks, however, have their own limitations. When standing still, the static force of the backpack is simply equal to the weight of the backpack. However, peak forces exerted on the body can increase dramatically to as much as 2-3-fold greater than the static force when one starts to walk or run while wearing the backpack. This increase is due to the requisite alternate deceleration and reacceleration of the load which must track the vertical movement of the hips on every step. These high, sometimes jarring, peak forces make it difficult to move at high speeds with large loads, and may also contribute to the muscular and orthopedic injury suffered by those carrying heavy weights relative to their body mass. In addition, there is a large increase in metabolic rate associated with walking (or running) with a load.
The hip rises 5-7 cm on each step. In a normal backpack, that would raise the load by 5-7 cm on each step. Raising the load requires that the frame exert an accelerative force on the load, and the load, in turn, exerts a downward force on the shoulders and body. This downward force also leads to increased ground reaction force pushing up at the foot-ground contact, and thereby increases force on the joints. The peak force can increase considerably. FIGS. 1A and 1B show that during walking at a fast pace, the peak force can increase to almost 2-fold larger (1.7 fold is shown) than the static or average force. During running, the peak force increases to 3-fold over the static force (FIGS. 2A and 2B).
This increase in force has large consequences. In the case of running, the increased force puts very high forces on the joints resulting in orthopedic and muscle pain and injury. A similar effect has been seen in soldiers with backpacks jumping out of trucks, during which the high forces incurred can lead to broken ankles. To eliminate these accelerative forces, it is necessary to keep the load from moving in the vertical plane to reduce dynamic forces on the body.
Because of our reliance on backpacks to carry loads, greater economy, lower dynamic forces and somewhat faster speeds (as well as potentially greater endurance), can have a wide range of health and societal benefits. The large weight of backpacks carried by children is well recognized to be a significant international public health problem, and reduction in dynamic forces may reduce muscle and orthopedic injury. The reduction in dynamic forces and improved economy are important to adults, particularly those carrying very heavy loads such as first responders, disaster relief workers, fire fighters, explorers, field scientists, and soldiers. Furthermore, in some of these cases, the ability to carry equipment to the disaster/emergency site faster could be the difference between success and failure. Finally, a more joint-friendly manner to carry smaller weights in young, middle-aged and aged populations, may reduce orthopedic injury to walkers, hikers, runners, golfers, and other athletes, thereby permitting lifelong mobility, and hence better health.
A backpack addressing many of these needs has been described in the aforementioned parent applications from which the present application claims priority. Subsequent research has revealed additional attributes desirable for a commercially viable ergonomic suspended-load backpack. The present application is directed to an ergonomic backpack having such attributes.